The parietal cortex is critically important for the control of spatial attention. In humans, unilateral damage to the parietal lobe can cause severe contralateral neglect - a syndrome characterized by failure to notice and orient toward objects contralateral to the lesioned hemisphere. The difficulties in neglect represent deficits of attention rather than sensory or motor deficits. In non-human primates, research has focused primarily on the role of the parietal cortex in the control of movement, in particular exploratory movements of the eye, arm or hand. Much less is known about the parietal mechanisms of attention and vision. The long-term goal of research in my laboratory is to determine the parietal mechanisms underlying vision and attention independently of movement. The proposed experiments will use neurophysiological experiments in conjunction with behavioral testing in awake, behaving monkeys. In some experiments neural activity is recorded while monkeys perform psychophysical tasks designed to measure attention. In other experiments, minute amounts of drugs are injected into the area analyzed, which temporarily silence this area, and the behavioral effects of such transient inactivation are measured. These experiments will allow us to characterize the neural activity associated with visual attention and the importance of a particular set of neurons for directing attention. We will concentrate on a subdivision of monkey posterior parietal cortex known as the lateral intraparietal area. Although evidence suggests that this area is important for vision and attention, its specific contributions have never been investigated. The experiments are designed to determine the impact of this parietal area on three aspects of vision that are strongly dependent on attention: the detection of isolated objects within the field of view (Aim 1); the discrimination of individual objects in crowded visual environments (Aim 2); and the orienting of attention based on voluntary factors and physical salience (endogenous and exogenous cues, respectively; Aim 3). The results are expected to significantly advance our understanding of the neural mechanisms of attention and to bring us closer toward establishing an animal model of neglect.